This invention relates to catheters especially, but not exclusively, for intravascular sensors such as those which measure pO2, pCO2, pH and temperature.
Catheters are often used to introduce sensing elements into the body, particularly into a blood vessel. Usually a hollow needle is used for making an initial entry into the blood vessel. A wire guide is then passed into the needle and the needle is then withdrawn. A catheter can then be slid over the wire guide and introduced into the blood vessel. After removing the wire guide, a sensor, e.g. including one or more optical fibres are then passed into the catheter. Often such catheters have a small diameter and thin walls and kink easily. Should the catheter kink, the sensor may give a false reading or be damaged. Replacement of a damaged sensor is relatively expensive and while a change is made to a new sensor, monitoring of a patient""s condition is interrupted.
Various attempts have been made to render catheters or introducer sheaths resistant to kinking. One procedure is described in European patent application No. 0617977 which describes an introducer in which a helical coil is sandwiched between an inner tube of PTFE and an outer tube of heat formable polyamide resin. The outer tube is heat formed and compressed so that material passes between turns in the coil and forms a connection with the roughened outer surface of the inner tube. It is difficult to control the manufacture to cause material from the outer tube to flow between the turns of the coil without distorting the inner tube. Also the reinforced introducers described in the above reference are relatively thick because they consist of two tubes with a coil sandwiched between them.
One object of the invention is to provide a more reliable and simpler method of producing a kink-resistant catheter.
Another object is to produce a kink-resistant catheter consisting of a tube having a thin wall in which a reinforcing coil is embedded and a method of its production.
A related object is to provide a reinforced catheter suitable for introducing or guiding a sensor into human tissue, such as a blood vessel, while resisting kinking and protecting the sensor from damage should the catheter be disturbed.
Another object is to provide a simple and reliable method of providing a tapered end to the catheter.
Such sensors are typically constructed with sensing elements each of about 0.002 inches diameter, in the case of wire and 0.007 inches in diameter in the case of optical fibres. When sensing elements for all four parameters are present, the sensor has an overall diameter of approximately 0.02 inch (0.5 mm). This small size is necessary to allow the sensor to be placed in an artery through a cannula and not compromise the primary function of the cannula of monitoring blood pressure and allowing blood samples to be taken.
The small size of the sensors means that they are fragile. Bending of optical fibre sensors results in a change in the signals. Progress has been made in reducing the impact of this by providing a second, reference wavelength but until the advent of this invention, sensor kinking remained a problem.
According to one aspect of the invention there is provided a method of making a kink-resistant catheter, said method comprising the steps of:
(i) supporting a helical coil having a plurality of turns on a mandrel which extends though the coil and is covered with a release surface;
(ii) introducing the helical coil while supported on the mandrel into a first tube, said tube being formed from a material which is flowable at a first elevated temperature and said first tube being received within a second tube, said second tube being heat shrinkable at a second temperature which is equal to or greater than said first temperature;
(iii) heating the resulting assembly to a temperature equal to or greater than said second temperature, whereby shrinkage of the second tube causes material of said first tube to flow between turns of the coil and substantially encapsulate the coil; and
(iv) removing the mandrel, the release surface and the second tube.
In some embodiments of the invention the first tube comprises polyethylene or polyurethane and the second tube comprises polyester, PTFE or FEP. FEP can be the second, heat-shrinkable tube in cases where the first tube is flowable at temperatures at which FEP does not flow but is heat-shrinkable. The most preferred combination is to employ a first tube which is polyurethane and a second tube which is FEP. Materials which are chemically very similar, e.g. PTFE and FEP, should not be used in conjunction as the first and second tubes because they tend to bond together.
Preferably the first tube is heat flowable at a first temperature in the range 100 to 300xc2x0 C. (212 to 572xc2x0 F.) and the second tube is heat shrinkable at a second temperature equal to or greater than the first temperature and preferably in the range 150 to 350xc2x0 C. (302 to 662xc2x0 F.).
In the manufacture of the reinforced catheter, the helical coil can be mounted on a mandrel and then introduced into the first tube, which is received within the second tube. A release surface is provided on the mandrel to facilitate removal of the mandrel after formation of the reinforced catheter. Suitable release agents include liquid release agents such as silicone oils. However, it is preferred, for two reasons, to use a solid release coating such as a PTFE or FEP tube, fitted onto the mandrel. First, it is found that when the mandrel carries a PTFE or FEP tube, the assembly does not adhere to the mandrel in the heating step and the mandrel can be easily drawn out from the assembly. Also, by using a PTFE or FEP sheathing tube on the mandrel, the flowable plastic of the first tube is effectively squeezed between the second tube and the inner release tube during the heating step. This enables higher temperatures to be employed which ensures that the plastic of the first tube flows to encapsulate the helical coil. At the same time, a second tube and the PTFE or FEP mandrel sheathing tube ensure that a smooth surface is formed on the inner and outer surfaces of the thus formed reinforced catheter. While a mandrel comprising a polished metal surface is satisfactory when used with a release agent such as a silicone oil, the presence of small scratches can cause adhesion and difficulty in removing the mandrel from the assembly.
However, the provision of a PTFE or FEP sheath tube on the mandrel can give rise to another difficulty in practice. This arises because, after removal of the mandrel, the sheathing tube is found to have adhered to the inside of the assembly. The Applicants have found that a reliable way of removing the sheathing tube in these circumstances is to apply a longitudinal tensioning force to the sheathing tube. This is best done by making the PTFE sheathing tube longer than the assembly of first and second tubes and helical coil so that the sheathing tube extends beyond one or, preferably both, ends of the assembly. Thus, after the heating step, the sheathing tube can be tensioned, preferably by pulling it outwardly at both ends. This action reliably breaks the adhesion with the inside of the assembly so that the sheathing tube can be removed.
Preferably, the second tube is also removed after the heating step. This is conveniently accomplished by slitting the tube lengthwise.
In this way, a catheter of small diameter and very thin wall thickness is obtained, which nevertheless is highly resistant to kinking.
In some embodiments of the invention the tubes extend beyond one or both ends of the helical coil.
The invention also encompasses a method of making a kink-resistant catheter which method comprises forming an assembly by the steps of:
(i) supporting a helical metallic coil having a plurality of turns on a mandrel which has a release surface comprising a polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene copolymer (FEP) sheathing tube supported on the mandrel;
(ii) introducing the helical coil while supported on the mandrel into a first tube, said first tube being formed from a material which is flowable at a first elevated temperature and said first tube being received within a second tube, said second tube being heat shrinkable at a second temperature which is equal to or greater than said first temperature;
(iii) heating the resulting assembly to a temperature equal to or greater than said second temperature, whereby shrinkage of the second tube causes material of said first tube to flow between turns of the coil and substantially encapsulate the coil;
(iv) removing the mandrel while leaving the sheathing tube within the coil; and
(v) removing the sheathing tube by tensioning said tube longitudinally of the coil.
According to the invention there is further provided a catheter which comprises a helical metallic coil having a plurality of longitudinally spaced apart turns, said coil being substantially encapsulated in a matrix comprising a tubular member of heat-flowable material which extends between turns of the coil, said tubular member having an exposed outer surface and an inner surface covering said spaced apart turns, and wherein the coil terminates inwardly of at least one end of the tubular member and wherein an inwardly tapered tip is present at said end.
The helical coil is metallic, for example, stainless steel and preferably the coils have a rounded cross-section, e.g. a circular cross-section. In some embodiments of the invention, a non-magnetic helical coil is used. An example is the metallic alloy known as MP35-N. Non-magnetic coils can be advantageous since adverse interactions with nuclear magnetic resonance systems, for example for imaging, can be reduced or eliminated.
The first tubular member may comprise polyethylene or polyurethane. The second tubular member may comprise PTFE, polyester or FEP.
Preferably the catheter has a tapered tip, the helical coil terminating inwardly of the tip. Provision of a tapered tip greatly improves the case of introducing the catheter into a blood vessel.
Advantageously, a reinforced catheter of very small wall thickness can be made by removing the second tube after the first tube has been caused to flow through turns in the helical coil. This can be readily achieved, for example, by providing a second tube which extends beyond an end of the first tube, forming a slit in the projecting end of the second tube after the heat shrinkage step and peeling the second tube away from the first. By selecting materials for the first and second tubes as described above, the first tube does not bond to the second and can easily be parted. The preferred procedure is to first remove the mandrel. Sometimes the sheathing tube comes away with the mandrel. More commonly, however, the mandrel is removed leaving the sheathing tube within the encapsulated helical coil. As mentioned above, the sheathing tube can be removed in these circumstances by applying a tension to opposite projecting ends of the sheathing tube.
After the sheathing tube has been removed, the inside diameter of the reinforced catheter is slightly larger so that a metal rod or wire support of correspondingly larger diameter is inserted into the bore of the catheter. The second tube is then removed, preferably by the technique described above, while supporting the assembly on the metal rod or wire support.
The reinforced catheters of the invention have the same or similar inside and outside diameters as conventional un-reinforced catheters but are extremely flexible and resistant to flattening when bent around a tight curve.